In ion based radiotherapy each ion will emit most of its energy towards the end of its path, creating what is known as the Bragg peak. A key issue in treatment planning is to ensure that the Bragg peaks of all beams is placed within the treatment volume, in such a way that all parts of the treatment volume receive the prescribed dose while minimizing dose to the surrounding volume.
The position of the Bragg peak is affected by the kinetic energy Tp of each ion. The values for Tp are selected so that the ions having the lowest energy will stop in an area at the nearest end of the treatment volume and the ions having the highest energy will stop in the area at the farthest end of the treatment volume.
In ion based radiotherapy the ions follow individual paths through the treatment volume from the point of incidence to the point where the ion has lost all its energy and stops. This point is referred to as the “track end”. In ion radiotherapy the distribution of track ends is of great interest as the track ends determine the end of range of the treatment field.
Current methods for delivering ion radiotherapy include:                The active scanning technique where the patient irradiation is delivered as a sequence of quasi-monoenergetic “spots” whose energy, direction and weight are selected so that the treatment volume is covered. The active technique is sometimes referred to as pencil beam scanning (PBS).        The passive technique, i.e. where the patient is irradiated by broad fields where the direction, incident energy and lateral extension is modulated so that the treatment volume is covered. The passive technique can be realized by several technical solutions. Examples are double scattering (DS), uniform scanning (US) or wobbling.        
The present invention is applicable to all current methods for delivering ion radiotherapy.
Throughout this document both the active scanning technique and the passive technique will be discussed. For the sake of simplicity, these will sometimes be referred to as just PBS and DS, respectively, as the most prominent examples of the respective technique.
For a PBS plan the spots in a beam can be grouped in “energy layers” during the process of creating the plan. All spots in an energy layer have the same incident energy spectrum. An energy layer can be assigned an index (the control point index) and a nominal energy (the control point energy).
For a DS plan the energy of the ions is delivered as single irradiation field per beam. In this field the ion energies are distributed over an energy range defined so that the Bragg peaks fall between the shallowest part and the most distal part of the treatment volume. The incident energy is further laterally range modulated by a compensator and collimated by an aperture.
For a DS plan there are usually no energy layers in the same way as there are for a PBS plan, but it is still possible for a passive technique plan to group the incident energies into energy layers.
It should be understood throughout this document that when describing the invention and when there is no distinction essential for the purpose of the invention needed between the active and passive techniques we will use the term energy layer in this broader sense.
There are uncertainty factors due to CT calibration, tissue inhomogeneity, organ motion and deformation. Because of such uncertainty factors there is a desire for a plan to be as robust as possible, meaning that it should provide the same dose distribution even if some factor changes. It is important to evaluate the quality of a radiotherapy treatment plan, to ensure that it will be delivered correctly and affect the patient in the desired way. When evaluating a radiotherapy treatment plan, its robustness is a key factor. The robustness reflects how well the plan will work in the case of small changes to the setup. For example, if the patient receiving the treatment moves relative to the assumed position this will affect where the particles will stop and thereby also the treatment.
Current methods for determining robustness include                displaying beam dose per energy layer. In this case the dose distribution for a single energy layer is shown for each image        displaying Bragg peak positions for the ions.        